1. Technical Field of the Invention
The present invention relates to a gas sensor element for sensing the concentration of a specific component in a gas to be measured (to be simply referred to as a measurement gas hereinafter).
2. Description of the Related Art
In the exhaust system of an internal combustion engine of a motor vehicle, there is generally arranged a gas sensor for sensing the concentration of a specific component (e.g., the concentration of oxygen) in the exhaust gas from the engine.
The gas sensor has, for example, a known gas sensor element built therein. The known gas sensor element includes: a solid electrolyte body having oxygen ion conductivity and an opposite pair of first and second surfaces; a measurement electrode provided on the first surface of the solid electrolyte body so as to be exposed to the measurement gas (i.e., the exhaust gas from the engine); a reference electrode provided on the second surface of the solid electrolyte body so as to be exposed to a reference gas (e.g., air); and a porous diffusion-resistant layer through which the measurement gas is introduced to the measurement electrode.
However, the known gas sensor element involves the following problem.
The gas sensor element has an outer surface to be exposed the flow of the exhaust gas from the engine. During startup of the engine, steam contained in the exhaust gas will condense into water droplets, and the water droplets will flow along with the exhaust gas toward the gas sensor element. The gas sensor element is generally used at high temperatures (e.g., not lower than 500° C.) at which the solid electrolyte body can be activated. Therefore, upon adherence of the water droplets to the outer surface of the gas sensor element, a large thermal shock may occur in the gas sensor element, thereby inducing cracking of the gas sensor element (to be referred to as water-induced cracking hereinafter).
To solve the above problem, Japanese Patent Application Publication No. 2006-171013 discloses a first technique, according to which a porous protective layer is provided at the outer periphery of the gas sensor element. Consequently, water droplets, which have adhered to the gas sensor element, can be dispersed into the porous protective layer, thereby preventing water-induced cracking of the gas sensor element.
However, with the first technique, to reliably prevent water-induced cracking of the gas sensor element, it is necessary to set the thickness of the porous protective layer sufficiently large. As a result, the heat capacity of the entire gas sensor element will be accordingly increased, thereby making it difficult to ensure both prompt activation and high responsiveness of the gas sensor element.
Japanese Patent Application Publication No. H8-240559 discloses a second technique, according to which a water-repellent surface layer is provided at the outer periphery of the gas sensor element. Consequently, water droplets, which approach the gas sensor element, can be repelled by the surface layer, thereby preventing water-induced cracking of the gas sensor element.
However, with the second technique, the surface layer is always water-repellent even at room temperature. Consequently, it may be difficult to accurately conduct a super-insulation inspection for the gas sensor element before use.
More specifically, the super-insulation inspection is generally conducted at room temperature by (1) immersing the gas sensor element in water or a liquid mixture of water and alcohol for a given time, thereby making water or the liquid mixture permeate into microcracks of the gas sensor element; (2) applying a predetermined voltage across the measurement and reference electrodes of the gas sensor element; and (3) measuring the electrical resistance between the two electrodes. However, since the surface layer of the gas sensor element is water-repellent at room temperature, it may be difficult to make water or the liquid mixture of water and alcohol sufficiently permeate into the microcracks of the gas sensor element. Consequently, it may be difficult to accurately conduct the super-insulation inspection for the gas sensor element.
Moreover, with the second technique, the surface layer is made of at least one of hydrophobic materials which include BN, CaF2, NbC, ZrB2, TiB2, and talc. However, it is easy for those hydrophobic materials to be oxidized at high temperatures. Consequently, in a lean atmosphere (i.e., an atmosphere containing a greater amount of oxygen), it may be difficult to accurately sense the concentration of the specific component in the measurement gas in an early stage, thus resulting in an increase in the activation time of the gas sensor element (i.e., the length of time from when the activation of the gas sensor element is started to when the gas sensor element becomes able to accurately sense the concentration of the specific component in the measurement gas). As a result, it may be difficult to ensure prompt activation of the gas sensor element.